1. Technical Field
The present invention relates to light-emitting devices, light-emitting apparatuses, display devices and electronic apparatuses.
2. Related Art
Organic electroluminescent devices (organic EL devices) are light-emitting devices having at least one luminescent organic layer interposed between an anode and a cathode. In this type of light-emitting device, electrons and holes are injected from the cathode side and the anode side, respectively, into the luminescent layer upon application of an electric field between the cathode and the anode. The electrons and the holes recombine within the luminescent layer to form excitons. The excitons then return to the ground state, releasing energy in the form of light.
Light-emitting devices which have two or more luminescent layers including a phosphorescent layer and a fluorescent layer between a cathode and an anode are known in the art.
In the case where such a light-emitting device is configured such that the phosphorescent layer and the fluorescent layer are stacked in contact with each other, the triplet energy of the phosphorescent layer is transferred toward the fluorescent layer and is thereafter deactivated without contributing to light emission, resulting in insufficient luminous efficiency.
In order to prevent or suppress the triplet energy transfer, it has been proposed that the phosphorescent layer and the fluorescent layer sandwich a single intermediate layer that contains both an electron transport material and a hole transport material, or an intermediate layer that satisfies a relation in terms of triplet energy with the phosphorescent layer and the fluorescent layer (see, for example, JP-A-2006-172762 and International Publication No. 2008/123178.
However, the provision of such an intermediate layer results in problems that the drive voltage of the light-emitting device is increased and that one of the phosphorescent layer and the fluorescent layer does not become luminous efficiently.